The present invention relates to a method and an apparatus for controlling the idle speed of an engine, and more particularly, to a method and an apparatus for controlling the idle speed of an engine by adjusting the ignition timing.
When idling, that is, when the engine is not loaded by the driver depressing the accelerator, engine idle speed control reflects a variety of engine operation factors. Typically, the engine idle speed is controlled using the intake air drawn into the engine in the idle state, or by controlling the ignition timing. The former method is usually used to cope with a large change in engine load that may be caused by, for example, changing the selector lever position of an automatic transmission. With a small change in the engine load, such as an electrical system load change, the ignition timing is usually controlled.
According to a conventional method for controlling idle speed by controlling the ignition timing, a deviation in the current engine idle speed is calculated relative to a target idle speed. Ignition timing is controlled based on whether the deviation is positive or negative. That is, the ignition timing is retarded when the current engine idle speed is higher than the target idle speed, and the ignition timing is advanced when the current engine idle speed is lower than the target idle speed. The amount of the ignition timing is retarded or advanced is usually proportional to the deviation. Thus, the ignition timing is adjusted based on only the engine speed deviation according to the prior art, so the response to an engine load change is slow and the response speed is not uniform. Why the response speed is not uniform is shown in FIG. 1.
FIG. 1 illustrates the ignition timing dependency of the output torque of an engine at a specific engine speed and engine load. The horizontal axis in FIG. 1 denotes ignition timing angle xcex8, and the vertical axis denotes torque ratio, that is, percentage of output torque with respect to the maximum torque at the engine speed and the engine load. As can be gathered from FIG. 1, at a specific engine speed and engine load, the output torque of an engine varies with respect to ignition timing. In FIG. 1, xe2x80x9cxcex8Axe2x80x9d denotes the ignition timing producing the maximum output torque, and xe2x80x9cxcex8Bxe2x80x9d denotes an ignition timing slightly advanced from xcex8A. The bands about xcex8B and xcex8A show that for the same amount of ignition timing adjustment, the output torque change is dependent on the current ignition timing. That is, when the current ignition timing is at the maximum-torque ignition timing xcex8A, the output torque change according to the ignition timing adjustment is very small relative to the same adjustment at xcex8B. Consequently, the engine speed change in the first case is very small.
Therefore, when the ignition timing is adjusted based on only the engine speed deviation without considering the corresponding torque ratio change, the resulting change in engine speed is not uniform. Furthermore, when the corresponding torque ratio change is small, the engine speed slowly approaches the target idle speed because the number of control iterations is increased.
A preferred embodiment of an apparatus for controlling the idle speed of an engine according to the present invention includes: an engine speed detector for detecting an engine speed; an engine load detector for detecting an engine load; a vehicle speed detector for detecting a vehicle speed; a coolant temperature detector for detecting a coolant temperature; an air temperature detector for detecting an intake air temperature; an ignition system for igniting fuel in the engine; and a control unit for receiving signals from the detectors and for activating the ignition system based on the received signals. The control unit executes instructions for steps of a preferred embodiment of a method for controlling the idle speed of an engine according to the present invention. In a further preferred embodiment, a knock detector for detecting knocking of the engine is included.
A preferred embodiment of a method for controlling the idle speed of an engine according to the present invention includes: detecting an engine speed; detecting an engine load; calculating a target torque ratio on the basis of the engine speed and the engine load using a predetermined relationship between ignition timing and torque ratio; calculating a target ignition timing corresponding to the target torque ratio using the predetermined relationship; and activating an ignition system of the engine based on the target ignition timing. In a further preferable embodiment, the calculating a target torque ratio includes: calculating a base torque ratio based on the engine speed and the engine load; and calculating the target torque ratio from the base torque ratio.
In another preferable embodiment, the calculating a base torque ratio includes: calculating a first torque ratio based on the engine speed and engine load; calculating a first ignition timing corresponding to the first torque ratio using the predetermined relationship; adjusting the first ignition timing based on intake air temperature and coolant temperature; and calculating the base torque ratio as corresponding to a base ignition timing using the predetermined relationship.
The first torque ratio is preferably calculated as a value within a range of about 85-90%, using a predetermined map table based on the engine speed and engine load.
With use of a knock detector, the calculating a base torque ratio may preferably further include: calculating adjusted values of the first ignition timing for individual cylinders based on signals from a knock detector; and adjusting the first ignition timing to a base ignition timing, wherein the base ignition timing is an average of the adjusted values for the individual cylinders.
In a further preferable embodiment, calculating the target torque ratio from the base torque ratio calculates the target torque ratio from the base torque ratio based on an engine speed deviation and an engine speed change rate. The calculating of the target torque ratio from the base torque ratio may preferably include: calculating a proportional gain based on the engine speed deviation; calculating a differential gain based on the engine speed change rate; calculating a coefficient based on the engine speed deviation; calculating a torque difference as a result of xe2x80x9cproportional gainxc3x97engine speed deviation+differential gainxc3x97engine speed change ratexc3x97coefficientxe2x80x9d; calculating a maximum torque at the engine speed and engine load; and calculating the target torque ratio as a sum of the base torque ratio and a torque value calculated based on the torque difference and the maximum torque.
In a further preferred embodiment, the method further includes determining if an idle speed control condition is satisfied, and the calculating a target torque ratio calculates the target torque ratio if the idle speed control condition is satisfied.
The satisfaction of the idle speed control condition is preferably determined based on at least whether a predetermined time has elapsed from starting the engine and whether a throttle valve is closed.
In a further preferred embodiment, the ignition timing of the predetermined relationship is a relative ignition timing based on a maximum-torque ignition timing, the calculating a target ignition timing includes: calculating a relative ignition timing corresponding to the target torque ratio using the predetermined relationship; calculating the maximum torque ignition timing, and calculating the target ignition timing as the maximum-torque ignition timing increased or decreased by the relative ignition timing. The relative ignition timing is preferably a retarded relative ignition timing from the maximum-torque ignition timing.